Disruption of thalamic connectivity figures prominently in hypotheses on the neural circuitry involved in schizophrenia. In contrast to robust evidence for such disruption from imaging studies, investigations on its underlying pathology lag far behind. Here, we focus on axonal pathways and myelinating cells within the mediodorsal nucleus (MD), a large thalamic nucleus interconnected with the frontal cortex through massive myelinated axon pathways and known to be involved in schizophrenia. We place our investigations in the context of emerging evidence for a role of chondroitin sulfate proteoglycans (CSPGs) in the pathophysiology of schizophrenia and in the regulation of brain connectivity and myelination. We have shown that the expression of CSPGs, main components of the extracellular matrix, is markedly altered in glial cells and perineuronal nets (PNNs; CSPG-enriched extracellular matrix structures surrounding distinct neuronal populations) in several brain regions of subjects with schizophrenia. The relevance of these findings to neural connectivity resides in the powerful role that CSPGs, and their interactions with oligodendrocyte progenitor cells (OPCs), play in axon guidance, fasciculation, myelination and impulse conduction. Preliminary results in human MD show axons enveloped by CSPG-enriched `axonal coats' and intimately associated with CSPG-expressing OPCs, as well as altered organization of myelinated fiber bundles in the MD of SZ subjects. Together, these considerations support the hypothesis that, in the MD of subjects with schizophrenia, altered CSPG expression in OPCs and axonal coats is associated with white matter/oligodendrocyte abnormalities and dysregulation of molecular pathways related to CSPGs and myelin biosynthesis and regulation. The proposed postmortem investigations will test this hypothesis using a combination of quantitative microscopy and proteomics/glycomics on MD samples from healthy control, schizophrenic and bipolar disorder subjects. Specific Aim 1 will elucidate the structure and composition of axonal coats, a novel extracellular matrix structure shown to surround axons in the human MD. Specific Aim 2 will use quantitative microscopy to test the hypothesis that, in the MD of subjects with schizophrenia, altered CSPG expression in OPCs and axonal coats is associated with disruption of myelination and oligodendrocyte reductions. Specific Aim 3 will use proteomics and glycomics analyses on the MD to test the hypothesis that CSPGs, myelin and molecular pathways related to their biosynthesis and regulation are disrupted in the MD of subjects with schizophrenia, thus setting microscopy studies in the context of focused hypotheses related to molecular mechanisms potentially responsible for abnormalities affecting CSPGs in axonal coats and OPCs. Specific Aim 4 will test the hypothesis that, in schizophrenia, CSPG/OPC/myelin abnormalities coexist with PNN decreases in a thalamic region, i.e. the reticular nucleus, which is particularly enriched in these ECM pericellular structures and plays a key role in gating prefrontal cortex-thalamus connectivity.